Existing examples of multiphase DC/DC converters of this type include a two-phase DC/DC converter 10 disclosed in U.S. Pat. No. 6,362,986, illustrated in FIG. 1.
In the two-phase DC/DC converter 10, input voltages Va and Vb are switched between by two switching devices 12 so that they are alternately converted into an output voltage Vc. A duty ratio, which is a ratio of the output voltage Vc to the input voltage Va or Vb is set to about 50% or between about 5% to 90%. The output voltage Vc is smoothed by two inductors, which are formed of windings 14A and 14B as an inductor array, and a capacitor 17. The windings 14A and 14B are wound about rungs 16A and 16B of a core 16, and the rungs 16A and 16B, together with risers 16C and 16D, form the core 16 which is substantially shaped like a square. When an input voltage Va is applied to the windings 14A and 14B through switching performed by the two switching devices 12 with a phase difference of about 180 degrees, magnetic flux 18A collides with magnetic flux 18C. When an input voltage Vb is applied to the windings 14A and 14B, magnetic flux 18B collides with magnetic flux 18D. Hence, when the input voltage Va or Vb is applied to the windings 14A and 14B, a magnetic flux flow in the core 16 is decreased, and negative coupling is generated between inductors formed by the windings 14A and 14B.
A magnetic inductance formed as a result of magnetic flux interlinking with the core 16 is set to be higher than or equal to about three times a leakage inductance formed as a result of magnetic flux leaking from the core 16. In other words, a coupling factor k between inductors formed by the windings 14A and 14B is substantially set to −0.75 or less (k≦−0.75). In the case where the duty ratio is about 50%, as shown by characteristics line 19 represented by a solid line, in the graph illustrated in FIG. 2, the smaller the coupling factor k with respect to this negative value (k≦−0.75) and, hence, the stronger the negative coupling, the smaller the ripple current in the inductors, whereby the DC voltage conversion efficiency of the two-phase DC/DC converter 10 increases. Note that the horizontal axis of the graph represents the coupling factor k and the vertical axis represents ripple current (A) in the inductors.